Multi-functionalization of a portable electronic device has proceeded faster than power saving of an electronic component. Therefore, the portable electronic device has been increased in power consumption. In consequence, a lithium ion battery serving as a main power supply of the portable electronic device has been required to have a high capacity and a small size more strongly than ever before. In addition, along with growing demand for an electric vehicle, also a lithium ion battery to be used in the electric vehicle has been strongly required to have a high capacity.
Graphite has hitherto been mainly used as a negative electrode material for the lithium ion battery. Graphite exhibits excellent cycle characteristics, but can stoichiometrically occlude lithium only up to a ratio of LiC6. Therefore, a theoretical capacity of graphite for a negative electrode is 372 mAh/g.
In order to realize a high capacity of the lithium ion battery, an investigation has been made on using particles containing a metal element having a high theoretical capacity, such as Si or Sn, for the negative electrode material. For example, the theoretical capacity of a lithium ion battery using particles containing Si for the negative electrode material is 4,200 mAh/g. The theoretical capacity of a lithium battery using metal lithium for a negative electrode is 3,900 mAh/g, and hence it is expected that a lithium ion battery having a smaller size and a higher capacity than those of the lithium battery is obtained when Si or the like can be used for the negative electrode material. However, the negative electrode material, such as Si, shows a high expansion rate and a high contraction rate in association with intercalation and deintercalation (occlusion and release) of lithium ions. Therefore, a capacity as high as that expected is not obtained owing to a gap generated between the particles. In addition, the particles are broken to be finer through repetition of great expansion and contraction. Therefore, electrical contact is disrupted and hence internal resistance increases. In consequence, the lithium ion battery to be obtained has a drawback of a short charge-discharge cycle lifetime.
In view of the foregoing, various composite negative electrode materials each combining a carbonaceous material and Si have been proposed. For example, there have been proposed: a composite material prepared by immobilizing Si ultrafine particles onto the surfaces of graphite particles, and mixing petroleum mesophase pitch therewith, followed by carbonization (Patent Document 1); a composite material prepared by mechanically pulverizing Si powder and natural graphite with a planetary ball mill to embed Si in the graphite, and then dissolving a carbon fiber and coal tar pitch in THF, followed by carbonization (Patent Document 2); a composite material prepared by mixing spherical natural graphite, Si, and PVA serving as a pore forming agent, and mixing binder pitch therewith under heating, followed by carbonization, and further mixing binder pitch and acetylene black therewith, followed by carbonization (Patent Document 3); a composite material prepared by mixing Si and powder pitch, and further dry-mixing artificial graphite therewith, followed by two-stage firing of tar removal at 600° C. and carbonization at 900° C. (Patent Document 4); and a composite material prepared by mixing a solution in which graphite is dispersed in xylene, a solution in which petroleum pitch is dispersed in xylene, and a solution in which pitch and Si are dispersed in xylene, followed by carbonization (Patent Document 5). Further, there has been proposed a production method of preparing a composite material by: mixing and kneading flake natural graphite, Si, and coal tar pitch in tar oil with a biaxial kneader; removing tar from the mixture at 450° C. and pulverizing the mixture; applying a compressive force and a shear force with Mechanofusion (trademark) system; and carbonizing the resultant at 1,000° C. (Patent Document 6).